Optical recording medium and method of producing the same

BACKGROUND OF THE INVENTION

0001The present invention relates generally to optical recording media and a method of producing the same, and more particularly, to an optical recording medium capable of having data printed on a label side opposite to a light incident side and a method of producing the same.

0002As recording media for recording large digital data, optical recording media represented by CDs (Compact Discs) and DVDs (Digital Versatile Discs) have widely been used and particularly optical recording media of such a type that data can be recorded by users are rapidly diffusing in recent years. As optical recording media capable of recording data are usable for preserving digital data such as image data and musical data having larger files simply at low cost, they have become utilized by a great deal of users. With the diffusion of optical recording media of the type mentioned above, there has increasingly developed a demand for making original optical recording media by having data printed on label sides opposite to light incident sides with printers and optical recording media capable of satisfying such a demand have already been developed and marketed.

0003An ink receiving layer for fixing ink is provided on the label side of an optical recording medium, so that printing can be done on the label side by using an ink-jet printer for supplying ink to the ink receiving layer.

0004However, printing quality for the optical recording medium is generally lower than what is for glossy paper and the problem is that the performance of the ink-jet printer capable of high-quality printing cannot be brought out satisfactorily. In order to solve this problem, there has been proposed an optical recording medium by JP-A-2002-237103 in which the surface roughness of an ink receiving layer is restrained from exceeding a predetermined value.

0005Although it is considered desirable to form such an ink receiving layer by using a spin coating method to reduce the surface roughness of the ink receiving layer, the surface roughness of the ink receiving layer thus formed is not always reducible even when an application liquid for making obtainable a flat smooth surface is selected because the surface roughness of the ink receiving layer is affected by the undercoat thereof and the problem is that high printing quality remains unavailable in this case.

0006Consequently, it has been not necessarily easy to reduce the surface roughness even in case that the ink receiving layer is formed by using the spin coating method.

0007On the other hand, there is a slit coating method known as a coating method capable of forming a coating film having a flat smooth surface and normally used for a rectangular shaped object (e.g., display panel) to be processed (refer to JP-A-11-162808 and JP-A-2000-167476). When the slit coating method is employed for disc-shaped objects to be processed like general optical recording media, the coating method brings with it a number of difficulties including causing a great step (difference in level) at the joint portion.

0008With the great step existing on the ink receiving layer, the step becomes conspicuous when printing is done with a printer, which poses a serious problem in that printing quality is deteriorated.

SUMMARY OF THE INVENTION

0009It is therefore an object of the invention to provide an optical recording medium wherein it is ensured that the surface roughness of an ink receiving layer is reduced whereby to make high-quality printing feasible, and a method of producing the same. It is another object of the invention to provide an optical recording medium wherein it is ensured that a step (difference in level) formed on the surface of an ink receiving layer is reduced whereby to make high-quality printing feasible, and a method of producing the same.

0010An optical recording medium, according to a first aspect of the invention, includes a disc body, an ink receiving layer provided on the label side of the disc body, and an undercoat layer provided between the label side of the disc body and the ink receiving layer, wherein the mean roughness (Ra) of the surface of the undercoat layer is not greater than 0.2 μm. Since the mean roughness (Ra) of the surface of the undercoat layer placed beneath the ink receiving layer is not greater than 0.2 μm according to the invention, it is ensured that the surface roughness of the ink receiving layer formed on the surface of the undercoat layer is reduced. Consequently, color development and gloss close to those of a film photo are obtainable when printing is done with an ink-jet printer.

0011Further, the mean roughness (Ra) of the surface of the undercoat layer is preferably not greater than 0.1 μm. Then higher printing quality is available because the roughness of the undercoat layer becomes almost unconfirmable through visual inspection.

0012Further, it is preferable that the undercoat layer includes at least a white ink layer. As color development is mainly improved thereby, enhanced printing quality becomes available.

0013A method of producing an optical recording medium, according to the first aspect of the invention, includes a first step of forming an undercoat layer by a screen printing method on the label side of a disc body so that the mean roughness (Ra) of the surface of the undercoat layer is not greater than 0.2 μm, and a second step of forming an ink receiving layer on the undercoat layer by a spin coating method or a slit coating method.

0014Further, a method of producing an optical recording medium includes a first step of forming an uncured undercoat layer containing ultraviolet-curable resin by a screen printing method on the label side of a disc body, a second step of curing the uncured undercoat layer by irradiating the undercoat layer with ultraviolet rays, and a third step of forming an ink receiving layer on the cured undercoat layer by a spin coating method or a slit coating method, wherein a duration of time required until the second step is taken after the completion of the first step is set longer than the time required to make the mean roughness (Ra) of the surface of the uncured undercoat layer not greater than 0.2 μm after the completion of the first step. With the arrangement above, the optical recording medium having the ink receiving layer with the reduced surface roughness becomes producible thereby.

0015Since the mean roughness (Ra) of the surface of the undercoat layer as the undercoat of the ink receiving layer is thus set not greater than 0.2 μm and preferably not greater than 0.1 μm according to the invention, it is ensured that the mean roughness (Ra) of the surface of the ink receiving layer is reducible. Consequently, color development and gloss close to those of a film photo are obtainable when printing is done with an ink-jet printer.

0016Further, an optical recording medium, according to a second aspect of the invention, includes a disc-shaped body, and at least an ink receiving layer which is provided on the label side of the disc body and has a step (difference in level) radially extended, wherein the step is not greater than 1.0 μm. Since the step on the ink receiving layer is not greater than 1.0 μm according to the invention, the step remains unconfirmable through visual inspection unless the step is observed from various angles when printing is done with the printer. Accordingly, high printing quality can be secured even though the ink receiving layer is formed by a method, such as the spin coating method, of causing a step (difference in level) to rise in the radial direction.

0017Further, the step is preferably not greater than 0.5 μm. On condition that the step is not greater than 0.5 μm, the step is hardly confirmable through visual inspection unless observed with special attention from various angles after printing is done with the printer, so that printing quality is almost never badly affected thereby.

0018It is preferable to further provide an undercoat layer between the label side of the disc body and the ink receiving layer. Printing quality can be enhanced further by providing the undercoat layer.

0019A method of producing an optical recording medium, according to the second aspect of the invention, includes a first step of forming a coating film by a slit coating method on the label side of a disc-shaped body, and a second step of forming an ink receiving layer by drying the coating film, is characterized in that a step produced at a joint portion where a liquid-application starting area and a liquid-application terminating area by using the slit coating method overlap each other is set not greater than 1.0 μm after the completion of the second step.

0020A method of producing an optical recording medium, according to the second aspect of the invention, includes a first step of forming a coating film on the label side of a disc-shaped body by rotationally moving the positional relation between a head having a slit for supplying an application liquid and the disc body, a second step of making a step has a gentle slop by turning the disc body in a joint portion where a liquid-application starting area and a liquid-application terminating area overlap each other, and a third step of forming an ink receiving layer by drying the coating film. In this case, the rotational time at the second step is set longer than the time required to make the step not greater than 0.1 μm after the completion of the third step. With the arrangement above, the optical recording medium having the ink receiving layer with the step with reduced difference in level becomes producible thereby.

0021As the step radially formed on the ink receiving layer is thus restrained to not greater than 1.0 μm and preferably not greater than 0.5 μm according to the invention, the step becomes inconspicuous when printing is done with the ink-jet printer, so that high printing quality can be secured.

BRIEF DESCRIPTION OF THE DRAWINGS

0022<figref idref="DRAWINGS">FIG. 1</figref> is a schematic sectional view of the structure of an optical recording medium <b>10</b> according to the preferred embodiment of the invention.

0023<figref idref="DRAWINGS">FIGS. 2A to 2C</figref> show examples of disc bodies <b>11</b> to which the invention is properly applicable, <figref idref="DRAWINGS">FIG. 2A</figref> is a CD type disc, <figref idref="DRAWINGS">FIG. 2B</figref> is a DVD type disc and <figref idref="DRAWINGS">FIG. 2C</figref> is a next-generation type disc.

0024<figref idref="DRAWINGS">FIGS. 3A and 3B</figref> are schematic depiction illustrating the way to form an ink receiving layer <b>13</b> on the disc-shaped body <b>11</b> by the slit coating method: <figref idref="DRAWINGS">FIG. 3A</figref> indicates the positional relation between the disc body <b>11</b> and a slit; and <figref idref="DRAWINGS">FIG. 3B</figref>, a coating area after the slit coating.

0025<figref idref="DRAWINGS">FIG. 4</figref> is a schematic sectional view taken on line A—A of <figref idref="DRAWINGS">FIG. 3B</figref>.

0026<figref idref="DRAWINGS">FIG. 5</figref> is a schematic plan view of a fan-shaped slit <b>50</b>.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

0027A description will now be described of the preferred embodiment of the invention with reference to the accompanying drawings.

0028<figref idref="DRAWINGS">FIG. 1</figref> is a schematic sectional view of the structure of an optical recording medium according to the preferred embodiment of the invention.

0029As shown in <figref idref="DRAWINGS">FIG. 1</figref>, an optical recording medium <b>10</b> according to the embodiment of the invention comprises a disc body <b>11</b>, a white ink layer <b>12</b> provided on the label side <b>11</b><i>b </i>of the disc body <b>11</b> and an ink receiving layer <b>13</b> provided on the white ink layer <b>12</b>. The white ink layer <b>12</b> is a layer formed as the undercoat of the ink receiving layer <b>13</b> and the layer provided between the label side <b>11</b><i>b </i>of the disc body <b>11</b> and the ink receiving layer <b>13</b> may be called a “undercoat layer”.

0030The disc body <b>11</b> has a light incident side <b>11</b><i>a </i>onto which a laser beam is emitted at the time of recording and/or playback and a label side <b>11</b><i>b </i>as the back of the light incident side <b>11</b><i>a. </i>Disc bodies <b>11</b> are not particularly restricted in kind but can be CD type discs such as CD-ROM, CD-R and CD-RW; DVD type discs such as DVD-ROM, DVD-R and DVD-RW; and further next-generation type optical discs for which a laser beam in the blue wavelength region is used.

0031<figref idref="DRAWINGS">FIGS. 2A to 2C</figref> show examples of disc bodies <b>11</b> to which the invention is properly applicable, including <figref idref="DRAWINGS">FIG. 2A</figref> is a CD type disc, <figref idref="DRAWINGS">FIG. 2B</figref> a DVD type disc and <figref idref="DRAWINGS">FIG. 2C</figref> a next-generation type disc.

0032As shown in <figref idref="DRAWINGS">FIG. 2A</figref>, the CD type disc has a light-transmission substrate <b>21</b> about 1.2 mm thick with its one side used as the light incident side <b>11</b><i>a, </i>a functional layer <b>22</b> provided on the other side of the light-transmission substrate <b>21</b> and a protective layer <b>23</b> covering the functional layer <b>22</b>, the surface of the protective layer <b>23</b> corresponding to the label side <b>11</b><i>b. </i>Accordingly, the white ink layer <b>12</b> and the ink receiving layer <b>13</b> are provided on the surface (label side <b>11</b><i>b</i>) of the protective layer <b>23</b> when the CD type disc shown in <figref idref="DRAWINGS">FIG. 2A</figref> is used as the disc body <b>11</b>. Generally, the structure and material of the functional layer <b>22</b> vary with the kind of the disc as follows: in the case of a ROM type disc such as CD-ROM, the functional layer <b>22</b> is formed with a reflective layer containing metal; in the case of a WO type disc such as CD-R, the functional layer <b>22</b> is formed with a recording layer containing organic coloring matter as well as a reflective layer containing metal, formed on the recording layer; and in the case of a rewritable disc such as CD-RW is formed with a recording layer containing phase-change material, a plurality of dielectric layers with the recording layer held therebetween and a reflective layer containing metal.

0033As shown in <figref idref="DRAWINGS">FIG. 2B</figref>, the DVD type disc has a light-transmission substrate <b>31</b> about 0.6 mm thick with its one side used as the light incident side <b>11</b><i>a, </i>a dummy substrate <b>32</b> with its one side used as the label side <b>11</b><i>b, </i>a functional layer <b>33</b> provided on the other side of the light-transmission substrate <b>31</b>, a protective layer <b>34</b> covering the functional layer <b>33</b>, and an adhesive layer <b>35</b> for bonding the laminate composed of the light-transmission substrate <b>31</b>, the functional layer <b>33</b> and the protective layer <b>34</b> to the adhesive layer <b>35</b>. Accordingly, the white ink layer <b>12</b> and the ink receiving layer <b>13</b> are provided on the surface (label side <b>11</b><i>b</i>) of the dummy substrate <b>32</b> when the DVD type disc shown in <figref idref="DRAWINGS">FIG. 2(</figref><i>b</i>) is used as the disc body <b>11</b>. Generally, as described above, the structure and material of the functional layer <b>33</b> vary with the kind of the disc as follows: in the case of a ROM type disc such as DVD-ROM like the CD type disc, the functional layer <b>33</b> is formed with a reflective layer containing metal; in the case of a WO type disc such as DVD-R, the functional layer <b>33</b> is formed with a recording layer containing organic coloring matter as well as a reflective layer containing metal, formed on the recording layer; and in the case of a rewritable disc such as DVD-RW is formed with a recording layer containing phase-change material, a plurality of dielectric layers with the recording layer held therebetween and a reflective layer containing metal.

0034As shown in <figref idref="DRAWINGS">FIG. 2C</figref>, the next-generation type disc has a support substrate <b>41</b> about 1.1 mm with its one end used as the label side <b>11</b><i>b, </i>a functional layer <b>42</b> provided on the other side of the support substrate <b>41</b> and a light-transmission substrate <b>43</b> covering the functional layer <b>42</b>, the surface of the light-transmission substrate <b>43</b> corresponding to the light incident side <b>11</b><i>a. </i>Accordingly, the white ink layer <b>12</b> and the ink receiving layer <b>13</b> are provided on the surface (label side <b>11</b><i>b</i>) of the support substrate <b>41</b> when the next-generation type disc shown in <figref idref="DRAWINGS">FIG. 2C</figref> is used as the disc body <b>11</b>. Generally, the structure and material of the functional layer <b>42</b> vary with the kind of the disc; in the case of a rewritable disc proposed now, the functional layer <b>42</b> is formed with a recording layer containing phase-change material, a plurality of dielectric layers with the recording layer held therebetween and a reflective layer containing metal.

0035Although the discs shown in <figref idref="DRAWINGS">FIGS. 2A to 2C</figref> are disc-shaped and about 1.2 mm in thickness and about 120 mm in diameter, discs to which the invention is applicable are not limited to those shown therein but may be one of any type as long as it has the light incident side <b>11</b><i>a </i>and the label side <b>11</b><i>b </i>opposite thereto. Moreover, the disc-like external form is not indispensable and not only the light incident side <b>11</b><i>a </i>but also the label side <b>11</b><i>b </i>may be rectangular.

0036The white ink layer <b>12</b> and the ink receiving layer <b>13</b> will be described next.

0037The white ink layer <b>12</b> is a white-colored layer (undercoat layer) as the undercoat of the ink receiving layer <b>13</b> and provided so as to improve printing quality by mainly improving the color development. According to the invention, the mean roughness (Ra) of the surface <b>12</b><i>a </i>of the white ink layer <b>12</b> is set not greater than 0.2 μm and preferably not greater than 0.1 μm. Although the thickness of the white ink layer <b>12</b> is not particularly restricted but may preferably be set not less than 8 μm and not greater than 15 μm. As the material of the white ink layer <b>12</b>, use can typically be made of material containing ultraviolet-curable resin and especially ultraviolet-curable acrylic resin containing not less than 10 wt % and not greater than 30 wt % of titanium oxide as well as having a lower shrinkage factor.

0038It is preferable to form the white ink layer <b>12</b> by the screen printing method. More specifically, it is preferable to cure the white ink layer <b>12</b> by irradiating the layer with ultraviolet rays after the uncured white ink layer <b>12</b> containing ultraviolet-curable resin is formed by the screen printing method. This is because the use of the screen printing method for forming the white ink layer <b>12</b> makes the film thickness substantially constant in the inner and outer peripheral portions as the film thickness distribution becomes reduced. In case that the film distribution of the white ink layer <b>12</b> is large, there is the possibility that a difference in color development is caused between the thin and thick areas of the white ink layer <b>12</b>.

0039When the white ink layer <b>12</b> is formed by the screen printing method, many irregularities corresponding to the screen mesh are formed on the surface <b>12</b><i>a </i>with the white ink layer <b>12</b> uncured immediately after the screen printing and the mean roughness (Ra) exceeds 0.2 μm and typically remains at about 0.3 μm. In order to smooth the irregularities and to set the mean roughness (Ra) of the surface <b>12</b><i>a </i>not greater than 0.2 μm, the leveling of the surface <b>12</b><i>a </i>is needed by placing time to some extent until the white ink layer <b>12</b> is cured by irradiating the layer with ultraviolet rays. The time required for leveling the surface <b>12</b><i>a </i>varies with the composition of the resin used to form the white ink layer <b>12</b>, the kind and quantity of a leveling agent to be added, whereupon a proper time corresponding to the requirement has to be set.

0040Although the mean roughness (Ra) of the surface <b>12</b><i>a </i>is made reducible simply by using the spin coating method for forming the white ink layer <b>12</b>, the use of the spin coating method may make a difference in film thickness between the inner and outer peripheral portions (e.g., resulting in a thin inner peripheral portion and a thick outer peripheral portion) and this is unpreferable because a difference in color development arises when printing is done with a printer.

0041The ink receiving layer <b>13</b> is a layer formed as the outermost layer on one side of the optical recording medium <b>10</b> and performs the role of fixing ink on receiving the ink supplied from an ink-jet printer. Printing quality, especially color development and gloss are improved when printing is done with the printer by minimizing the mean roughness (Ra) of the surface <b>13</b><i>a </i>of the ink receiving layer <b>13</b>. In order to obtain so-called photographic image quality in terms of color development and gloss close to the quality of a film photo, the mean roughness (Ra) of the surface <b>13</b><i>a </i>of the ink receiving layer <b>13</b> is required to be not greater than 0.2 μm and preferably not greater than 0.1 μm. The thickness of the ink receiving layer <b>13</b> is not particularly restricted but may preferably be set not less than 10 μm and not greater than 30 μm. The ink receiving layer <b>13</b> is preferably made of material with hydrophilic resin such as polyvinyl alcohol and polyvinyl acetal as the main ingredients mixed with cationic polymer as an ink fixing agent.

0042According to the invention, the spin coating method or the slit coating method is used to form the ink receiving layer <b>13</b>. As is well known, the spin coating method includes the steps of dropping an application liquid (a liquid prepared by diluting the material of the ink receiving layer <b>13</b> dissolved in a solvent with water or any other solvent) onto the center or the vicinity of the center of the surface (<b>12</b><i>a</i>) of an object to be processed and turning the processing object whereby to spread the application liquid in the outer peripheral direction by centrifugal force. On the other hand, the slit coating method includes the steps of supplying an application liquid from a slit provided in a head and spreading the application liquid over the surface of an object to be processed by moving the relative position between the head and the processing object. The slit coating method is a coating method normally used for the processing object (e.g., a display panel) having a rectangular surface and in case where this method is used for disc-shaped objects to be processed such as optical discs in general, a great deal of difficulty is accompanied thereby. Consequently, it is necessary to devise a way to deal with the case where the disc body <b>11</b> is disc-shaped.

0043The reason for the use of the spin coating method or the slit coating method for forming the ink receiving layer <b>13</b> is that the mean roughness (Ra) of the surface <b>13</b><i>a </i>can be made reducible by using one of these coating methods. Although the distribution of the film thickness of the ink receiving layer <b>13</b> tends to grow larger with the spin coating method or the slit coating method used in comparison with the use of the screen printing method, the printing quality is hardly affected by a modicum of the film thickness distribution since the ink receiving layer <b>13</b> is fundamentally transparent.

0044When the ink receiving layer <b>13</b> is formed by the spin coating method or the slit coating method, the surface property of the white ink layer <b>12</b> as the undercoat is greatly reflected on the formation of the ink receiving layer <b>13</b> unlike the case of using the screen printing method. In other words, the mean roughness (Ra) of the surface <b>12</b><i>a </i>of the white ink layer <b>12</b> directly appears as the mean roughness (Ra) of the surface <b>13</b><i>a </i>of the ink receiving layer <b>13</b>. Consequently, the mean roughness (Ra) of the surface <b>12</b><i>a </i>of the white ink layer <b>12</b> is set not greater than 0.2 μm according to the invention, whereby the mean roughness (Ra) of the surface <b>13</b><i>a </i>of the ink receiving layer <b>13</b> can also be set not greater than 0.2 μm.

0045As the mean roughness (Ra) of the surface <b>12</b><i>a </i>of the white ink layer <b>12</b> is not greater than 0.2 μm in the optical recording medium <b>10</b> thus configured, the mean roughness (Ra) of the surface <b>13</b><i>a </i>of the ink receiving layer <b>13</b> is sufficiently reducible. Accordingly, color development and gloss close to those of the film photo are obtainable when printing is done with the ink-jet printer.

0046A description will now be given of a method of forming the ink receiving layer <b>13</b> on the disc-shaped body <b>11</b> by the slit coating method.

0047<figref idref="DRAWINGS">FIGS. 3A and 3B</figref> are schematic depiction illustrating the way to form the ink receiving layer <b>13</b> on the disc-shaped body <b>11</b> by the slit coating method: <figref idref="DRAWINGS">FIG. 3A</figref> indicates the positional relation between the disc body <b>11</b> and the slit; and <figref idref="DRAWINGS">FIG. 3B</figref>, a coating area by means of slit coating.

0048As shown in <figref idref="DRAWINGS">FIG. 3A</figref>, the longer direction of a slit <b>50</b> is matched to the radial direction of the disc body <b>11</b> so that one end <b>51</b> of the slit <b>50</b> provided on a head (not shown) for use in supplying an application liquid matches with the innermost peripheral portion of a coating area and that the other end <b>52</b> of the slit <b>50</b> matches with the outermost peripheral portion of the coating area when a slit coat is applied to the disc-shaped body <b>11</b>. The disc body <b>11</b> is turned or the head is turned along the disc body <b>11</b> in this condition whereby to move the relative positional relation therebetween. As shown in <figref idref="DRAWINGS">FIG. 3B</figref>, the application liquid is spread in the form of a doughnut on the coating surface (surface <b>12</b><i>a </i>of the white ink layer <b>12</b>) and then the ink receiving layer <b>13</b> is formed by letting the solvent evaporate.

0049Since an object to be processed is disc-shaped, however, a liquid-application starting area substantially matches with a liquid-application ending area unlike the case of coating an object to be processed such as a display panel having a rectangular surface. Therefore, as shown in <figref idref="DRAWINGS">FIG. 3B</figref>, the film thickness of an area <b>60</b> forming a joint becomes slightly greater than the thickness of any other portion. In other words, there is produced a slight difference in level in the area <b>60</b> where the joint is formed. <figref idref="DRAWINGS">FIG. 4</figref> is a sectional view taken on line A—A of <figref idref="DRAWINGS">FIG. 3B</figref>, showing the condition above. As shown in <figref idref="DRAWINGS">FIG. 4</figref>, the film thickness T<b>1</b> of the area <b>60</b> as the joint and the mean film thickness T<b>2</b> on the periphery of the area <b>60</b> have a relationship of T<b>1</b>>T<b>2</b>, so that a step, that is difference in level therebetween, as defined by T<b>1</b>−T<b>2</b> arises.

0050In case that the step radially extended grows larger, the joint becomes conspicuous when printing is done with the printer, so that printing quality is badly affected thereby. In consideration of this fact, the step (=T<b>1</b>−T<b>2</b>) is set not greater than 1.0 μm and preferably not greater than 0.5 μm according to the invention. This is because the step remains unconfirmable through visual inspection unless the step is observed from various angles when printing is done with the printer on condition that it is not greater than 1.0 μm. In case that the step is not greater than 0.5 μm, the step is made unconfirmable through visual inspection unless it is observed with special attention from various angles after the printer is used for printing data, so that printing quality is almost not badly affected thereby.

0051When the coating film is dried immediately after slit coating is completed, such a step (difference in level) normally exceeds 1.0 μm and in order to make the step have a gentle slope up to a level not greater than 1.0 μm, the disc body <b>11</b> has to be turned for a certain interval of time until the coating film is heat-dried after slit coating is carried out so as to subject the surface <b>13</b><i>a </i>to leveling by centrifugal force. As the time required for leveling varies with the composition of the material used for the disc body <b>11</b>, the kind and quantity of the solvent used to dissolve the material and further the number of revolutions of the disc body <b>11</b>, the time required therefor has to be properly set. With respect to the number of revolutions of the disc body <b>11</b> at the leveling time, the number of revolutions has to be set smaller to the extent that the film thickness of the ink receiving layer <b>13</b> is not excessively decreased; for example, not less than 50 rpm and not greater than 300 rpm.

0052Since an object to be processed is disc-shaped, moreover, the object has a distinctive character in that the linear velocity of the head differs between the inner and outer peripheral portions of the coating surface. In case the application liquid supplied by the slit <b>50</b> is uniform from one end <b>51</b> corresponding to the innermost periphery over the other end <b>52</b> corresponding to the outermost periphery, it is likely that the ink receiving layer <b>13</b> formed becomes thick in the inner peripheral portion and becomes thin in the outer peripheral portion. When the printing quality is not affected by the difference in film thickness above, no problem is offered but it is preferable that the distribution of the film thickness of the ink receiving layer <b>13</b> is restrained by increasing the amount of application liquid supplied from one end <b>51</b> corresponding to the innermost periphery to the other end <b>52</b> corresponding to the outermost periphery; in order to realize this, the profile of the slit <b>50</b> is fan-shaped as shown in <figref idref="DRAWINGS">FIG. 5</figref> by setting the slit width W<b>2</b> at the other end <b>52</b> corresponding to the outermost peripheral portion greater than the slit width W<b>1</b> at one end <b>51</b> corresponding to the innermost peripheral portion of an area to be coated therewith.

0053When the ink receiving layer <b>13</b> is formed by the slit coating method, the surface property of the white ink layer <b>12</b> as the undercoat is greatly reflected on the formation of the ink receiving layer <b>13</b> unlike the case of using the screen printing method. In other words, the mean roughness (Ra) of the surface <b>12</b><i>a </i>of the white ink layer <b>12</b> directly appears as the mean roughness (Ra) of the surface <b>13</b><i>a </i>of the ink receiving layer <b>13</b>. Consequently, the mean roughness (Ra) of the surface <b>12</b><i>a </i>of the white ink layer <b>12</b> is preferably set not greater than 0.2 μm, whereby the mean roughness (Ra) of the surface <b>13</b><i>a </i>of the ink receiving layer <b>13</b> can also be set not greater than 0.2 μm.

0054The optical recording medium <b>10</b> according to the embodiment of the invention is thus formed by the producing method as described above.

0055In the optical recording medium <b>10</b> thus formed, the step in the area <b>60</b> as the joint of the ink receiving layer <b>13</b> is restrained to not greater than 1.0 μm, so that the joint becomes substantially inconspicuous when printing is done with the printer. Moreover, the mean roughness (Ra) of the surface <b>13</b><i>a </i>of the ink receiving layer <b>13</b> is made sufficiently reducible by setting the mean roughness (Ra) of the surface <b>12</b><i>a </i>of the white ink layer <b>12</b> not greater than 0.2 μm, whereby color development and gloss close to those of the film photo are obtainable when printing is done with the ink-jet printer.

0056As set forth above, the mean roughness (Ra) of the surface <b>12</b><i>a </i>of the white ink layer <b>12</b> as the undercoat of the ink receiving layer <b>13</b> is set not greater than 0.2 μm and preferably not greater than 0.1 μm to ensure that the mean roughness (Ra) of the surface <b>13</b><i>a </i>of the ink receiving layer <b>13</b> formed by the spin coating method or the slit coating method is made reducible thereby. Consequently, color development and gloss close to those of the film photo are obtainable when printing is done with the ink-jet printer.

0057Further, since the step at the joint area <b>60</b> of the ink receiving layer <b>13</b> formed by the slit coating method according to the embodiment of the invention is restrained to not greater than 1.0 μm and preferably not greater than 0.5 μm, the join becomes inconspicuous even when printing is done with the ink-jet printer, so that high printing quality is available.

0058The invention is not limited to the embodiment described above but may include various changes and modifications within the scope of the invention as hereinafter claimed and such changes and modifications are needless to say intended to be within the scope thereof.

0059For example, though the white ink layer <b>12</b> is formed directly on the label side <b>11</b><i>b </i>of the disc body <b>11</b> in the optical recording medium <b>10</b> according to the above embodiment of the invention, another layer may be placed between the disc body <b>11</b> and the white ink layer <b>12</b>. Although another layer can be placed between the white ink layer <b>12</b> and the ink receiving layer <b>13</b>, further, the mean roughness (Ra) of the layer placed beneath the ink receiving layer <b>13</b> has to be not greater than 0.2 μm in this case. In other words, the mean roughness (Ra) of the surface of the undercoat layer existing between the label side <b>11</b><i>b </i>of the disc body <b>11</b> and the ink receiving layer <b>13</b> is only needed to be set not greater than 0.2 μm.

0060For example, though the white ink layer <b>12</b> is formed directly on the label side <b>11</b><i>b </i>of the disc body <b>11</b> in the optical recording medium <b>10</b> according to the above embodiment of the invention, another layer may be placed between the disc body <b>11</b> and the white ink layer <b>12</b>. Although another layer can be placed between the white ink layer <b>12</b> and the ink receiving layer <b>13</b>, the mean roughness (Ra) of the surface of the layer placed beneath the ink receiving layer <b>13</b> is preferably not greater than 0.2 μm in this case. In other words, the mean roughness (Ra) of the surface of the undercoat layer existing between the label side <b>11</b><i>b </i>of the disc body <b>11</b> and the ink receiving layer <b>13</b> is preferably not greater than 0.2 μm.

0061Although the step is made to have a gentle slope by turning the disc body <b>11</b> after the coating film is formed by the slit coating method, the coating film may undergo natural leveling not by turning the disc body <b>11</b> but by simply leaving the coating film as it is. However, it is preferred for the leveling to be achieved by turning the disc body as in the above embodiment of the invention when it is taken into consideration that time allowing the natural leveling to be achieved is limited and that the leveling requires time.

EXAMPLES

0062A description will now be given of examples of the invention, which is not limited in any way to the following examples thereof, however.

Sample Making

Example 1

0063A sample of an optical recording medium having the same structure as that of the optical recording medium <b>10</b> shown in <figref idref="DRAWINGS">FIG. 1</figref> was produced by the following method. With respect the disc body <b>11</b>, a disc of the DVD structure shown in <figref idref="DRAWINGS">FIG. 2B</figref> was used.

0064First, an ultraviolet-curable resin about 10 μm thick was formed on the surface (label side <b>11</b><i>b</i>) of a dummy substrate <b>32</b> by screen printing using a screen mesh of No. #420. As the material of the ultraviolet-curable resin, Seika-Beam SCR-VID F29 white of Dainichiseika Color &amp; Chemicals Mfg. Co., Ltd was used.

0065Then the ultraviolet-curable resin thus formed was irradiated with ultraviolet rays to form a white ink layer <b>12</b> by curing the resin. A duration of five minutes was set from the screen printing up to ultraviolet radiation. Further, the surface roughness of the surface <b>12</b><i>a </i>of the white ink layer <b>12</b> was measured by using Surfcorder SE-3400 of Kosaka Laboratory Ltd. As a result, the mean roughness (Ra) of the surface <b>12</b><i>a </i>of the white ink layer <b>12</b> was 0.026 μm and the maximum roughness (Ry) thereof was 0.18 μm.

0066An application liquid containing 10–15 wt % of PVA (polyvinyl alcohol), 75–85 wt % of water, 5–10 wt % of IPA (isopropyl alcohol) and not greater than 5 wt % of other material was dropped by the spin coating method and applied to the surface <b>12</b><i>a </i>of the white ink layer <b>12</b> by turning the disc body <b>11</b>. Then the coating film was dried at 80° C. for five minutes to form an ink receiving layer <b>13</b> about 10 μm thick thereby. The shake-off number of revolutions in spin coating was set to 400 rpm. Then the Surfcorder SE-3400 mentioned above was used to measure the surface roughness of the surface <b>13</b><i>a </i>of the ink receiving layer <b>13</b>. As a result, the mean roughness (Ra) of the surface <b>13</b><i>a </i>of the ink receiving layer <b>13</b> was 0.026 μm and the maximum roughness (Ry) thereof was 0.18 μm.

0067Thus, the sample according to Example 1 of the invention was completed.

Example 2

0068An example 2 of the invention was produced as in example 1 thereof except that when the white ink layer <b>12</b> was formed, a duration of three minutes was set from the screen printing up to ultraviolet radiation. As a result, the mean roughness (Ra) of the surface <b>12</b><i>a </i>of the white ink layer <b>12</b> was 0.032 μm and the maximum roughness (Ry) thereof was 0.21 μm. Moreover, the mean roughness (Ra) of the surface <b>13</b><i>a </i>of the ink receiving layer <b>13</b> was 0.030 μm and the maximum roughness (Ry) thereof was 0.20 μm.

Example 3

0069An example 3 of the invention was produced as in example 1 thereof except that when the white ink layer <b>12</b> was formed, a duration of one minute and 30 seconds was set from the screen printing up to ultraviolet radiation. As a result, the mean roughness (Ra) of the surface <b>12</b><i>a </i>of the white ink layer <b>12</b> was 0.09 μm and the maximum roughness (Ry) thereof was 0.38 μm. Moreover, the mean roughness (Ra) of the surface <b>13</b><i>a </i>of the ink receiving layer <b>13</b> was 0.08 μm and the maximum roughness (Ry) thereof was 0.28 μm

Example 4

0070An example 4 of the invention was produced as in example 1 thereof except that when the white ink layer <b>12</b> was formed, a duration of one minute was set from the screen printing up to ultraviolet radiation. As a result, the mean roughness (Ra) of the surface <b>12</b><i>a </i>of the white ink layer <b>12</b> was 0.11 μm and the maximum roughness (Ry) thereof was 0.62 μm. Moreover, the mean roughness (Ra) of the surface <b>13</b><i>a </i>of the ink receiving layer <b>13</b> was 0.09 μm and the maximum roughness (Ry) thereof was 0.41 μm.

Comparative Example 1

0071A comparative example 1 was produced as in example 1 of the invention except that when a white ink layer <b>12</b> was formed, a duration of 30 seconds was set from the screen printing up to ultraviolet radiation. As a result, the mean roughness (Ra) of the surface <b>12</b><i>a </i>of the white ink layer <b>12</b> was 0.21 μm and the maximum roughness (Ry) thereof was 0.94 μm. Moreover, the mean roughness (Ra) of the surface <b>13</b><i>a </i>of the ink receiving layer <b>13</b> was 0.14 μm and the maximum roughness (Ry) thereof was 0.87 μm.

Comparative Example 2

0072A comparative example 2 was produced as in example 1 of the invention except that when the white ink layer <b>12</b> was formed, a duration of 2 seconds was set from the screen printing up to ultraviolet radiation. As a result, the mean roughness (Ra) of the surface <b>12</b><i>a </i>of the white ink layer <b>12</b> was 0.28 μm and the maximum roughness (Ry) thereof was 1.24 μm. Moreover, the mean roughness (Ra) of the surface <b>13</b><i>a </i>of the ink receiving layer <b>13</b> was 0.19 μm and the maximum roughness (Ry) thereof was 0.95 μm.

Evaluation of Samples

0073Printing was actually done on the ink receiving layers <b>13</b> in the examples 1–4 and those of comparative examples 1–2 by using a printer BJF950i of Canon Inc. by adjusting the paper setting to ‘printable disc (recommended product),’ the printing quality to ‘standard’ and the cartridge to ‘photo.’ Then the printing quality (color development, gloss and mesh traces) of the those samples was evaluated through visual inspection. Table 1 shows the results. The duration of time from the screen printing up to ultraviolet radiation, the mean roughness (Ra) of the surface <b>12</b><i>a </i>of the white ink layer <b>12</b> and the maximum roughness (Ry) thereof are shown collectively in Table 1.

0074<tables id="TABLE-US-00001" num="00001"><table frame="none" colsep="0" rowsep="0" pgwide="1"><tgroup align="left" colsep="0" rowsep="0" cols="7"><colspec colname="offset" colwidth="70pt" align="left" /><colspec colname="1" colwidth="42pt" align="center" /><colspec colname="2" colwidth="42pt" align="center" /><colspec colname="3" colwidth="35pt" align="center" /><colspec colname="4" colwidth="35pt" align="center" /><colspec colname="5" colwidth="35pt" align="center" /><colspec colname="6" colwidth="35pt" align="center" /><thead><row><entry /><entry namest="offset" nameend="6" rowsep="1">TABLE 1</entry></row><row><entry /><entry namest="offset" nameend="6" align="center" rowsep="1" /></row><row><entry /><entry>Comparative</entry><entry>Comparative</entry><entry /><entry /><entry /><entry /></row><row><entry /><entry>example 2</entry><entry>example 1</entry><entry>Example 4</entry><entry>Example 3</entry><entry>Example 2</entry><entry>Example 1</entry></row><row><entry /><entry namest="offset" nameend="6" align="center" rowsep="1" /></row></thead><tbody valign="top"><row><entry /></row></tbody></tgroup><tgroup align="left" colsep="0" rowsep="0" cols="7"><colspec colname="1" colwidth="70pt" align="left" /><colspec colname="2" colwidth="42pt" align="center" /><colspec colname="3" colwidth="42pt" align="center" /><colspec colname="4" colwidth="35pt" align="center" /><colspec colname="5" colwidth="35pt" align="center" /><colspec colname="6" colwidth="35pt" align="center" /><colspec colname="7" colwidth="35pt" align="center" /><tbody valign="top"><row><entry>Duration of time</entry><entry>2 sec.</entry><entry>30 sec.</entry><entry>1 min.</entry><entry>1.3 min.</entry><entry>3 min.</entry><entry>5 min.</entry></row><row><entry>from screen printing</entry></row><row><entry>up to ultraviolet</entry></row><row><entry>radiation</entry></row><row><entry>Ra (μm)</entry><entry>0.28</entry><entry>0.21</entry><entry>0.11</entry><entry>0.09</entry><entry>0.032</entry><entry>0.026</entry></row><row><entry>Ry (μm)</entry><entry>1.24</entry><entry>0.94</entry><entry>0.62</entry><entry>0.38</entry><entry>0.21 </entry><entry>0.18 </entry></row><row><entry>Texture · gloss</entry><entry>Δ</entry><entry>Δ</entry><entry>◯</entry><entry>◯</entry><entry>◯</entry><entry>◯</entry></row><row><entry>Undercoat roughness</entry><entry>x</entry><entry>Δ</entry><entry>Δ</entry><entry>◯</entry><entry>◯</entry><entry>◯</entry></row><row><entry namest="1" nameend="7" align="center" rowsep="1" /></row><row><entry namest="1" nameend="7" align="left" id="FOO-00001">Texture · gloss: Δ = ordinary, ◯ = good</entry></row><row><entry namest="1" nameend="7" align="left" id="FOO-00002">Undercoat roughness: x = readily confirmable, Δ = confirmable and ◯ = unconfirmable.</entry></row></tbody></tgroup></table></tables>

0075As shown in Table 1, color development and gloss were very good in the samples according to examples 1–4 of the invention in which the mean roughness (Ra) of the surfaces <b>12</b><i>a </i>of the white ink layers <b>12</b> was not greater than 0.2 μm. Particularly in the samples according to examples 1–3 of the invention in which the mean roughness (Ra) of the surfaces <b>12</b><i>a </i>of the white ink layers <b>12</b> was not greater than 0.1 μm, the roughness of the white ink layers <b>12</b> became unconfirmable through visual inspection, so that the printing quality was substantially equal to that of the film photo.

0076On the contrary, comparative examples 1 and 2 in which the mean roughness (Ra) of the surfaces <b>12</b><i>a </i>of the white ink layers <b>12</b> exceeded 0.2 μm were obviously inferior in color development as well as gloss to the samples according to examples 1–4 of the invention and the roughness of the white ink layers <b>12</b> became readily confirmable.

Example 5

0077An example of an optical recording medium having the same structure as that of the optical recording medium <b>10</b> shown in <figref idref="DRAWINGS">FIG. 1</figref> was produced by the following method. With respect the disc body <b>11</b>, a disc of the DVD structure shown in <figref idref="DRAWINGS">FIG. 2B</figref> was used.

0078First, an ultraviolet-curable resin about 10 μm thick was formed on the surface (label side <b>11</b><i>b</i>) of a dummy substrate <b>32</b> by screen printing using a screen mesh of No. #420. As the material of the ultraviolet-curable resin, Seika-Beam SCR-VID F29 white of Dainichiseika Color &amp; Chemicals Mfg. Co., Ltd was used.

0079Then the ultraviolet-curable resin thus formed was irradiated with ultraviolet rays to form a white ink layer <b>12</b> by curing the resin. A duration of one minute was set from the screen printing up to ultraviolet radiation.

0080Further, a head having a fan-shaped slit having a slit width W1 of 0.08 mm at one end <b>51</b> corresponding to the innermost peripheral portion and a slit width W2 of 0.12 mm at the other end <b>52</b> corresponding to the outermost peripheral portion is used. The head is placed closer up to 0.90 mm to the surface <b>12</b><i>a </i>of a white ink layer <b>12</b> and by turning a disc body <b>11</b> at 14 rpm in this condition, an application liquid (having a viscosity of 500 cps) containing 10–15 wt % of PVA (polyvinyl alcohol), 75–85 wt % of water, 5–10 wt % of IPA (isopropyl alcohol) and not greater than 5 wt % of other material was applied by slit coating to the surface <b>12</b><i>a </i>of the white ink layer <b>12</b> within a 20–58 mm radius from the center of the disc body <b>11</b>. Then the coating film was subjected to leveling by turning the disc body <b>11</b> at 100 rpm for 15 seconds before being dried at 80° C. for five minutes whereby to form an ink receiving layer <b>13</b> about 10 μm thick.

0081Thus, the example 5 of the invention was completed.

0082Further, ETA-RT of STEAG ETA-OPTIC GmbH Co. was used to measure the coating film of the ink receiving layer <b>13</b> at a 40 mm radial point by an optical interference method. As a result, the film thickness of an area <b>60</b> as a joint portion was 10.2 μm and the mean roughness (Ra) thereof on the periphery of the area <b>60</b> was 9.8 μm.

Example 6

0083An example 6 of the invention was produced as in example 5 thereof except that when the ink receiving layer <b>13</b> was formed, a duration of 10 seconds was set as leveling time by turning. As a result, the film thickness of the ink receiving layer <b>13</b> at the 40 mm radial point was 10.6 μm in the area <b>60</b> as the joint portion and the mean film thickness thereof on the periphery of the area <b>60</b> was 10.1 μm.

Example 7

0084An example 7 of the invention was produced as in example 5 thereof except that when the ink receiving layer <b>13</b> was formed, a duration of six seconds was set as leveling time by turning. As a result, the film thickness of the ink receiving layer <b>13</b> at the 40 mm radial point was 10.6 μm in the area <b>60</b> as the joint portion and the mean film thickness thereof on the periphery of the area <b>60</b> was 9.8 μm.

Comparative Example 3

0085A comparative example 3 was produced as in example 5 thereof except that when an ink receiving layer <b>13</b> was formed, a duration of four seconds was set as leveling time by turning. As a result, the film thickness of the ink receiving layer <b>13</b> at the 40 mm radial point was 11.6 μm in the area <b>60</b> as the joint portion and the mean film thickness thereof on the periphery of the area <b>60</b> was 10.2 μm.

Comparative Example 4

0086A comparative example 4 was produced as in example 5 thereof except that when the ink receiving layer <b>13</b> was formed, a duration of two seconds was set as leveling time by turning. As a result, the film thickness of the ink receiving layer <b>13</b> at the 40 mm radial point was 12.4 μm in the area <b>60</b> as the joint portion and the mean film thickness thereof on the periphery of the area <b>60</b> was 10.1 μm.

[Evaluation of Samples]

0087Printing was actually done on the ink receiving layers <b>13</b> in the samples according to examples 5–7 and those of comparative examples 3 and 4 by using the printer BJF950i of Canon Inc. by adjusting the paper setting to ‘printable disc (recommended product),’ the printing quality to ‘standard’ and the cartridge to ‘photo.’ Then the joint portions of those samples were evaluated through visual inspection. Table 2 shows the results. The film thickness (=T<b>1</b>) of the ink receiving layers <b>13</b> in the areas <b>60</b> as the joint portions, the mean film thickness (=T<b>2</b>) of the ink receiving layers <b>13</b> on the periphery of the areas <b>60</b> and differences in the film thickness (steps) (=T<b>1</b>−T<b>1</b>) thereof are shown collectively in Table 2.

0088<tables id="TABLE-US-00002" num="00002"><table frame="none" colsep="0" rowsep="0" pgwide="1"><tgroup align="left" colsep="0" rowsep="0" cols="6"><colspec colname="offset" colwidth="98pt" align="left" /><colspec colname="1" colwidth="42pt" align="center" /><colspec colname="2" colwidth="42pt" align="center" /><colspec colname="3" colwidth="35pt" align="center" /><colspec colname="4" colwidth="35pt" align="center" /><colspec colname="5" colwidth="35pt" align="center" /><thead><row><entry /><entry namest="offset" nameend="5" rowsep="1">TABLE 2</entry></row><row><entry /><entry namest="offset" nameend="5" align="center" rowsep="1" /></row><row><entry /><entry>Comparative</entry><entry>Comparative</entry><entry /><entry /><entry /></row><row><entry /><entry>example 2</entry><entry>example 1</entry><entry>Example 3</entry><entry>Example 2</entry><entry>Example 1</entry></row><row><entry /><entry namest="offset" nameend="5" align="center" rowsep="1" /></row></thead><tbody valign="top"><row><entry /></row></tbody></tgroup><tgroup align="left" colsep="0" rowsep="0" cols="6"><colspec colname="1" colwidth="98pt" align="left" /><colspec colname="2" colwidth="42pt" align="center" /><colspec colname="3" colwidth="42pt" align="center" /><colspec colname="4" colwidth="35pt" align="center" /><colspec colname="5" colwidth="35pt" align="center" /><colspec colname="6" colwidth="35pt" align="center" /><tbody valign="top"><row><entry>Leveling time by turning</entry><entry>2 sec.</entry><entry>4 sec.</entry><entry>6 sec.</entry><entry>10 sec.</entry><entry>15 sec</entry></row><row><entry>Film thickness (μm) of ink</entry><entry>12.4</entry><entry>11.6</entry><entry>10.6 </entry><entry>10.6</entry><entry>10.2 </entry></row><row><entry>receiving layer 13 in area 60</entry></row><row><entry>Mean film thickness (μm) of ink</entry><entry>10.1</entry><entry>10.2</entry><entry>9.8</entry><entry>10.1</entry><entry>9.8</entry></row><row><entry>receiving layer 13 in peripheral</entry></row><row><entry>area of area 60</entry></row><row><entry>Difference in film</entry><entry> 2.3</entry><entry> 1.4</entry><entry>0.8</entry><entry> 0.5</entry><entry>0.4</entry></row><row><entry>thickness (step)</entry></row><row><entry>External appearance</entry><entry>x</entry><entry>x</entry><entry>◯</entry><entry>⊚</entry><entry>⊚</entry></row><row><entry>of joint</entry></row><row><entry namest="1" nameend="6" align="center" rowsep="1" /></row><row><entry namest="1" nameend="6" align="left" id="FOO-00003">External appearance of joint: x = readily confirmable; ◯ = confirmable if observed carefully; ⊚ = substantially unconfirmable.</entry></row></tbody></tgroup></table></tables>

0089As shown in Table 2, the joint portions were substantially inconspicuous whereby to secure high printing quality in the samples according to Embodiments 5–7 of the invention in which differences in the film thickness (steps) were not greater than 1.0 μm. Particularly in the samples according to Embodiments 1–2 of the invention in which the differences in the film thickness (steps) were not greater than 0.5 μm, the joint portions became unconfirmable unless carefully observed, so that printing quality was substantially equal to that of the film photo.

0090In the samples of Comparative Examples 3–4 in which differences in the film thickness (steps) of the ink receiving layers <b>13</b> exceeded 1.0 μm, on the contrary, the joint portions became readily confirmable without being observed carefully.